First off, what year is your Taurus?
Second, have you tried opening other doors? Do any of them cause the interior light to come on? If other doors do, but the driver's door does not cause the interior light to illuminate, the door switch is probably the problem. How you fix it will depend on the year of your Taurus.
The door latches with integrated door pin switches commonly stop working. If you search this forum, you will find many suggestions on how to fix them without replacing the latch mechanism. I suggest a proper electrical parts cleaner rather than the common WD-40. WD-40 is not an electrical parts cleaner, and a door pin switch is an electrical part. That being said, many people have had good luck with WD-40. I used Caig DeOxit electrical parts cleaner on the faulty switch of my '98 1.5 years ago and it hasn't acted up again since. Again, others have had similar luck with the more readily available WD-40 (but I still don't recommend it).
-Rod